Various problems dealing with the production, isolation, differentiation, structural elucidation, biogenesis, and structural modification of different polyene macrolide antifungal antibiotics are currently under investigation. The principal emphasis and aim of all of these studies are the comparative determinations of the tertiary molecular structures of representative members of the heptaene, hexaene, pentaene and tetraene macrolide antifungal antibiotics. The major approach to the determination of these structures is the application of single crystal x-ray analysis. Fermentation procedures leading to the production of complex mixtures of unstable polyene macrolide components are followed by detailed chemical and physical studies designed to produce pure single components and crystalline derivatives suitable for single crystal x-ray analysis. Particular emphasis is placed on the comparative study of the composition of different polyene macrolide preparations, especially those employed in biological-medical investigations. A variety of liquid-liquid and thin layer chromatographic procedures as well as extensive countercurrent distribution studies are employed for the identification and purification and differentiation of particular polyene macrolide components. Studies dealing with the chemical modification of polyene macrolide structures and the resultant effect on their biological-antifungal action are also currently underway.